1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission having a reverse, drive and neutral control valve, and more particularly to a hydraulic control system which ensures the prevention of a shock which may occur at the time of an "N-R" AND an "N-D" manual shift, and can unify shift control pressure with one valve in case of an "N-R" and an "N-D" manual shift and a first-second speed shift.
2. Description of Related Art
A conventional automatic transmission has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter, which includes hydraulically actuated friction members for selecting one of the gear stages of the transmission gear mechanism in accordance with vehicle operating conditions.
The hydraulic control system pressurized by a fluid pump provides the working pressure required to operate the friction members and control valves.
A commonly used automatic transmission of a vehicle has a fluid torque converter which generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the hydraulic fluid from the turbine runner to a direction where the fluid flow does not disturb the rotation of the pump impeller when the fluid flows into the pump impeller.
An automatic shift is made by the operation of friction members such as clutches or a kick-down brake at each shift change. Also, a manual valve of which a port is converted by selecting a position of a selector lever, is designed to be supplied with fluid from a fluid pump and to supply the fluid to a shift control valve. In a 4-speed automatic transmission, the shift control valve has an operated port made by an electronic control system.
One example of a hydraulic pressure control system of an automatic transmission for a vehicle will be described in FIG. 4 which shows a circuit diagram of a conventional hydraulic control system comprising a torque converter 1 attached to an engine through the engine flexplate, and rotating at engine speed for transmitting power of the engine to an input shaft of the transmission gear mechanism, a damper clutch control valve 2 for controlling the application and release of a damper clutch to increase the power train efficiency inside the torque converter 1, a regulator valve 5 for regulating the output oil pressure of the fluid pump 4 according to the automatic transmission requirements, and a reducing valve 6 for regulating stably the supply of oil pressure to a solenoid valve and the damper clutch control valve 2.
A manual valve 7, which is connected to an outlet of the fluid pump 4 and is provided with the hydraulic pressure, is designed to deliver line pressure to the regulator valve 5 and a shift control valve. In the manual valve 7, lands are changed according to the position of a shift selector lever.
A shift control valve 8, which is operated in response to two shift control solenoid valves A and B controlled by a TCU is designed to transmit the oil pressure selectively through a first-second speed shift valve 9, an end clutch valve 10, a second-third and a third-fourth speed shift valve 11, and a rear clutch exhaust valve 12 to a front clutch 13, a rear clutch 14, a low and reverse brake 15, a kick down servo brake 16, an end clutch 17, and the like. An N-D control valve 18 is connected to the rear clutch 14. An N-R control valve 19 is connected to the first-second speed shift valve so as to reduce the impact caused by the shift.
Also, a pressure control solenoid valve 20 is connected to a pressure control valve 21 to reduce the shock produced by the control at the time of shifting.
The conventional hydraulic control system includes an N-R control valve and an N-D control valve to reduce the shift impact while is manually shifting from a neutral "N" range to a drive "D" range or a reverse "R" range. In this case, since the line pressure is controlled by a regulator valve, the N-R and N-D control valves, the control pressure becomes NON-linear and the system has a complicated structure.
When the control pressure is not linear, the shift impact occurs at the time between either manual shifting of N-R of and N-D. The N-R and N-D control valves are separately installed, so that the hydraulic circuit is complicated.